Money Pulse: compact numbers roll over into the wrong unit ($1000, 1000.0k) #344

Context

src/shared/pulse-format.js is the single source of truth for how numbers render on the Money Pulse page (src/pulse.js) and the viability panels (src/shared/viability-panels.js). Its compact formatters pick a unit by comparing the raw value to a threshold, then round. When rounding pushes a value up past the threshold, the label comes out in the wrong unit:

Verified on main:

node --input-type=module -e "
import { fmtUsd, fmtThree } from './src/shared/pulse-format.js';
console.log(fmtUsd(9.999), fmtUsd(999.6), fmtUsd(999999), fmtThree(999.6), fmtThree(999950));
"

$10.00 $1000 $1000.0k 1000 1000.0k

On a live counter these are exactly the numbers that appear right before a milestone, which is when people screenshot the page.

There are no tests for this file today.

What to change

Fix fmtUsd and fmtThree so the unit is chosen after rounding (or so the thresholds account for it), and give fmtUsd a millions branch. Keep every other output unchanged. Add tests/pulse-format.test.js covering the boundaries.

fmtSol, fmtNum, fmtPct, and the HTML helpers are out of scope for this issue.

Acceptance criteria

• Every "Expected" value in the table holds.
• Values well inside each band are unchanged (for example fmtUsd(0), fmtUsd(5), fmtUsd(42), fmtUsd(12345), fmtThree(0.5), fmtThree(42), fmtThree(340), fmtThree(12400), fmtThree(1200000)), locked in by tests.
• Each boundary (10, 1,000, 1,000,000) is tested just below, at, and just above.
• Non-numbers and negatives render the same as today.

How to test

npx vitest run tests/pulse-format.test.js
npm test
npm run dev

With the dev server running, open http://localhost:3000/pulse and confirm the counters still render.

Good first issue: pure functions, a visible result, and a clear table of right answers. Comment "taking this" before you start.